The present invention relates to electronic components, and particularly relates to items of manufacture for shielding electronic components from electromagnetic interference.
Background: EMI Shielding in PIUs
Electronic devices are susceptible to EMI, or Electromagnetic Interference. EMI is radiated or conducted energy that originates from electromagnetic sources, usually other electronic devices. EMI interferes with the normal operation of many electronic devices. One method of reducing the effects of EMI is to shield electronic components from the electromagnetic fields.
EMI shielding is usually achieved by creating a grounded closed shield of conductive material around the components to be protected. This effectively creates a region wherein electromagnetic fields, and hence EMI, are intercepted. Electric fields from inside do not radiate outward beyond the shielding, and exterior electric fields do not enter the protected area.
EMI shielding is applied in many areas. Particularly, plug-in units (PIUs) used in telephone switching require EMI shielding. PIUs are assemblies of printed circuit boards, electronic components, connectors, and a faceplate with latches. The PIUs combine with a backplate circuit board within the shelf to form an electronic system.
Background: Gaskets
PIUs are inserted into slots designed to fit the unit. Card holders are carriages (usually metal) that hold the PIUs in place. Gaskets are used to form a peripheral seal along the edges of a faceplate attached to the end of a PIU, which provides safety grounding of metal parts to the chassis. Latches on the faceplate secure the PIU within the shelf. The gaskets must be flexible enough to be insertable and still hold the carriage in place after insertion.
Another important function of a gasket is to provide a continuous metal connection so that electrical components within the PIU are shielded from EMI. Gaps in the connection between the PIU and the slot can be created when the gasket is damaged, or when the gasket no longer fits properly (such as when a spring experiences compression set or is damaged in other ways). These gaps, or discontinuities in the current flow in the shield, allow exterior fields to interfere with internal components and allow leakage of electromagnetic energy from the internal components to the environment.
Metal finger style EMI gaskets are fragile and easily damaged when not handled properly. This damage can create metal fragments from the spring fingers which might be introduced to sensitive areas, such as where circuitry is housed, causing shorts and other electrical and mechanical problems. Damaged gaskets can also cause snags, poor fit, or possibly injury to handlers.
Prior art attempts to remedy these problems have centered on replacing metal finger-type gaskets with soft-type gaskets. These solutions have their own problems, such as short material life, difficulty in mounting the gaskets, poorer EMI shielding than metal gaskets, flammability, and compression set. It is therefore desirable to have a metal gasket (offering superior shielding and grounding effects) which does not have the disadvantages of fragility, fragment loss, and compression set.
Conductive Fabric Wrapped Over Spring EMI Gasket
The present application discloses that spring gaskets can be improved by covering the springs with a pliable, conductive material, such as a metallized fabric. The covering adheres to a metal or plastic spring by an adhesive or by other means (such as thermal welding, for example).
Embodiments of the present teachings include metal or plastic springs covered in a woven polymer fabric that has been metallized on one or both sides. The adhesive used to fix the fabric to the spring are doped with conducting materials to make the adhesive itself a better conductor.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
fragile metal or plastic springs are protected from damage that can occur during use;
if the springs do fracture, the pieces are retained by the covering and adhesive, which prevents fragments from being introduced into electrical equipment or causing other problems;
the present innovations allow the use of a metal spring core (rather than foam, plastic, elastomer, or other material), which has better EMI shielding and grounding properties, and which experiences less degradation such as particle loss and compression set;
fabric covering decreases spring snagging, prolonging spring life;
fabric coverings may be added to existing springs, and altered springs are easily retrofitted to existing gaskets and holders, as well as existing designs.